Remove CONSTRAINT syntax for KEY | INDEX | FULLTEXT

[tangenta]

Index Issue: pingcap/tidb#11486

These SQL cases do not work in MySQL, but they work in TiDB parser:

create table t (a int, constraint c key idx (a))
create table t (a int, constraint c index idx (a))
create table t (a int, constraint c key idx using hash (a))
create table t (a int, constraint c index idx using rtree (a))
create table t (a int, constraint c fulltext index (a))

ConstraintKeywordOpt is not applicable in KEY, INDEX, or FULLTEXT according to the grammar of MySQL 8.0.

Claim this subtask based on the comments below this issue.

[wty4427300]

Let me fix it